A federal study that details the proposed expansion of passenger rail service in the Northeast, with Springfield serving as a key connection, was praised by local advocates Wednesday as a project with great promise and benefits to the region and state. The three-prong expansion of passenger rail service in the Northeast would serve an estimated 323,000 riders annually in Massachusetts alone, according to the study known as the Northern New England Intercity Rail Initiative (NNEIRI), citing 2035 ridership estimates.